[wxqc] Barometric pressure errors

Wed May 16 10:08:35 CDT 2007

Hi Chris,
Last year one of list participants wrote a great guide to resolving this
problem. I cleared up my errors and I be that it would do the same for you.
Please follow these directions step by step.
Good luck,
Rick  w0lta/ar797

Shameless self promotion.

www.w0lta.net

http://w0lta.net/weather/Current_Vantage_Pro.htm

On Sun 7/23/2006 7:46 PM - Evan Bookbinder [Evan.Bookbinder at noaa.gov] wrote

Subject - [wxqc] Davis VP barometer clarification

Obviously, there is still a lot of confusion out there from what exactly

needs to be done to get the Davis VP2 barometer correct both internally and

in terms of transmitting to CWOP/APRS. I hope these instructions will remove

a lot of the QC problems we're seeing.

Console:

- Set your elevation in the console to the elevation of the console

itself. For example, my house at ground level is 968 feet, and the console

is sitting in my 2nd floor office next to the computer -- at a height of 980

feet.

- Although indoor temperature flucutations are generally minor, let the

unit sit for a good 12-14 hours.

- Next, adjust the console as necessary to the SEA LEVEL PRESSURE of the

nearest official observation site (or an interpolation between two). Note

that I said SEA LEVEL PRESSURE, as this is what is displayed on the console

(a very useful meteorological value). How do we do this? Here's the easiest

way I've found.

1.) http://www.uswx.com/us/wx/

2.) Click on your state

3.) On the left hand side menu, click the purple "Stations" link

4.) You will see a list of all the official weather stations in your

state. Click on the 4 letter identifier for the observation sites closest to

your location.

5.) Making sure that the observation is current, you will see the

"Pressure" item in the table, which correctly uses the Sea Level Pressure

for that site. They display the pressure in both millibars (which I prefer

to display on my console) or inches of mercury. Your console should be

adjusted to THIS number (or interpolated between surrounding sites). It's

always recommended that you pick a day with light wind, which should

correspond to similar pressure readings in your area.

6.) NOTE: DO NOT use the Axxxx group in a coded METAR observation, or

the pressure given by calling into an airport observation site. These are

altimeter settings, and not what is required on the Davis VP2 console.

- From here on out, your console should run smoothly in tandem with the

official observation sites. My console is generally dead on or about 0.1 mb

different from the nearby airport site, exactly what we want to see.

CWOP/APRS:

Now, for whatever reason, CWOP/APRS uses altimeter setting, NOT Sea

Level Pressure as noted above. This may be due to the limitation of other

manufacturer's instruments reporting into the network. In any event, and as

discussed in previous threads, these are not one in the same. Altimeter

setting is primarily used for aviation purposes, while sea level pressure

(adjusted for temperature) is used for meteorological purposes.

Now, theoretically, you should have to do nothing further here. The

software you are using to send CWOP/APRS data *should* internally calculate

the altimeter setting from your console's sea level pressure data and

transmit it.

Again, MAKE SURE THAT YOU ARE USING SOFTWARE THAT SENDS ALTIMETER

SETTING DATA. If you are unclear, contact the software's author and make

sure that they are calculating this correctly.

VPLive works fantastic, as does WeatherLink 5.7.

***** For those using WeatherLink 5.6 or lower (which is a fairly common

problem from what I've seen), PLEASE make sure that you upgrade to V5.7

immediately. http://www.davisnet.com/support/weather/software.asp#vantagePro

*****

Only V5.7 correctly sends altimeter setting. V5.6 and earlier versions

incorrectly sent the sea level pressure directly from the console.

As a plea to Davis, in the APRS data string, it would really be nice to

see the software version number included with the generic "DsVP" in future

updates. This really makes troubleshooting a lot easier -- for example

"DsVP57".

Respectfully,

Evan Bookbinder

Senior Meteorologist

NWS Kansas City/Pleasant Hill MO

  -----Original Message-----
  From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net]On Behalf Of [hris }{olmes
  Sent: Sunday, May 13, 2007 12:58 PM
  To: 'Discussion of weather data quality issues'
  Subject: Re: [wxqc] Barometric pressure errors

  First of all my apologies to the group. I had intended to send my email to
John CW3212 and not everyone on this list. Sorry! John would have understood
my ramblings in that he had the same problem two years ago
http://server.gladstonefamily.net/pipermail/wxqc/2005-May/002898.html

"My QC reports have been showing that my barometric pressure readings drift
to either 3 millibars too high, or 3 millibars too low, and then track at
the same error." I had initially set my up my barometer per Davis's manual
using "Sea Level" Pressure from the nearest airport. In my case KBJC
http://www.met.utah.edu/cgi-bin/droman/meso_table_mesowest.cgi?stn=KBJC&unit
=0&time=GMTI had assumed some internal calculations in the Vantage Pro 2
Console would compensate for the large difference (Sea level and
Altimeter).This was shown to be incorrect after a few hours. I then used the
proper Altimeter reading. After a couple of days my readings would go up and
then down, following the pressure trend, but would be exaggerated by 3 or 4
mbs in whatever direction the barometer was heading.Having read that some
barometer errors were corrected in the latest firmware, I decided to try an
upload to the latest version even though I may have already had this. The
upload caused my barometer to "stick" and not change at all. Davis support
was aware of the problem and instructed me to set the elevation to zero and
let it go over night. That did not work but by deleting the zero and leaving
it blank did un-stick the barometer. Two days of observing the barometer,
elevation at sea level and adding the aprox. 5.5 inches to compensate for
5454 feet seemed to work.I have now started over with the correct elevation
and correct Altitude reading. Time will tell if this takes care of the
problem. I did read where one person had claimed he could tell who had a
Vantage Pro by just looking at the graphs of the various CWOP sites and
observing this error. I hope that is not the case. Chris  AR893

----------------------------------------------------------------------------
--

  From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of Don Jeerings
  Sent: Saturday, May 12, 2007 8:12 PM
  To: Discussion of weather data quality issues
  Subject: [wxqc] Barometric pressure errors

  Searching the "Net" to try to figure out why my brand new Vantage Pro 2
barometer tracks 3 or 4 mb's plus or minis over a period of a few days. I
ran across your inquire as to the same problem.  Notice your quality checks
are 2 thumbs up now. May I ask what did you finely do to correct this, set
to zero elevation? I am at 5454ft.

  My old weather monitor II was just set to zero and I added 5.35 inches
through software for years with great results.

  I found lots of references but no solutions.

  I would greatly like to know what you ended up doing.

  Chris

  AR893 (station off line because of the errors)

  Chris

  There may be two different problems, Keep in mind that corrections can be
made in the Vantage Pro 2 (ISS) console  or sometimes in the associated
software. (like WD Display software by Brian Hamilton).

  It is best to program the Vantage Pro 2 equipment first.

  You will need to first program in your altitude and since you are over
5,000 ft, this will make a significant correction.

  Once you have the altitude programmed in you can make small corrections by
using the BAR section.

  If you have the Davis Console Manual I will refer to the page numbers.

  If you do not have the manual, E-mail me back and I will try and copy out
the pertinent information for you.

   After you go into setup, look on Page 15  for the elevation correction
procedures,

   And look at the top of page 26 for making the barometer correction
procedures..

  Note that if you use the Mill bar units for you're BAR settings, you will
obtain better resolution  and better accuracy. (also use Fahrenheit on your
temperature for better resolution)

  After you make a calibration change you should wait at least 24 hours to
evaluate it. Make small changes and allow a day or so between calibration
corrections. KEEP A LOG of your corrections so you know where you are.

  Let me know if this helps,. I can copy down in detail and send you more
information.

  Don Jeerings

  Djeerings at tampabay.rr.com

  Note also that the set up is for programming of many other important
variables for your system.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://server.gladstonefamily.net/pipermail/wxqc/attachments/20070516/a9428547/attachment-0001.htm 